The best route for mass transit in Los Angeles is down Wilshire Boulevard. According to the LA Weekly, in an article I discussed here, this has not yet happened because the wealthy residents along much of Wilshire oppose this for class reasons. Their elected representative Henry Waxman, although primarily notable for a career of good government watchdogging, has duly enacted this opposition in the form of a law blocking subway development in a specified area around Wilshire. The premise of the law is that underground methane pockets make safe tunnelling impossible, but the real reason is class-based NIMBYism. The good news is that somebody has made a deal or twisted Waxman's arm enough to finally change his stand:

But new research has convinced the Los Angeles Democrat that tunneling can be done safely and he has introduced legislation to lift the ban. "We need a transit system. We have to alleviate the congestion," he said.